I didn't notice any showers at the TXL LH lounge, since it is not a major gateway and thus passengers have little use of such a facility. And as noted by DN, you would not be able to access the LH lounge since you were not flying them or a STAR partner. Depending upon your connection time at LHR between arrival at T4 and departure from T1, you may have time to use BA's Arrivals Lounge at T4, or one of the many showers at the facility at T1.